Cathode structure for thermionic devices



Aug. 29; 1933.

A. W. HULL CATHODE STRUCTURE FOR THERMIONIC DEVICES v Filed Oct. 9, 1930IIIIIIII DUI lilllllIIllOl Illll Olllli 5 1,} T F M i En i) u xv W L.1... 1,.,.. 1 nnnnhu n, v

wit l I I I l I 1 Inventor-z Albert w Hull,

His Attorney.

Patented Aug. 29, 1933 CATHODE STRUCTURE FOR THERMIONIC DEVICES AlbertW. Hull, Schenectady, N. Y., assignor to General Electric Company, aCorporation-of New York Application October 9, 1930. Serial No. 487,533-Claims. (01. 250-275) The present invention relates to electricaldischarge apparatus and more particularly to thermionic devices of thepower type. This application is a continuation in part of my applica- 5tion, Serial No. 226,276, filed October 15,, 1927, Patent 1,790,153,entitled Electrical discharge devices and method of operation. I

In the application referred to, there are described, inter alia, variousforms of cathode structures for electrical discharge devices of the arctype capable of handling large amounts of electrical energy. Devices ofthis sort contain an appreciable amount of gas or vapor at a pressuresufficient to support an arc-like discharge at the impressed potentialsand have a negative voltampere characteristic in which the current isnearly independent of the voltage. The cathodes of these devices areusually indirectly heated and employ a heater positioned within ametallic member which serves as the cathode proper. The

latter ordinarily is coated with an alkaline earth substance, such asbarium oxide to increase the electron emission thereof.

The maximum amount of current that a device of this kind can carry is afunction of the maximum electron emission from the cathode which, ioragiven emission per unit area, depends upon the total cathode area. It isdesirable therefore, in the case of large current output to provide thecathode with the greatest amount of electron emitting surface, at thesame time with the minimum bulk in order to facilitate the rapid heatingthereof by avoiding undue heat radiation.

The object of the present invention is to provide an improved form ofcathode in which the electron emitting area is enhanced without aproportional increase in cathode volume. This object is attained, inbrief, by providing the cathode with a plurality of radially projectingdiscs or vanes which are rendered thermionically active.

For a more complete understanding of the invention reference should nowbe made to the drawing in which Fig. 1 represents an electrostaticallycontrolled arc discharge device embodying the improved cathode; Fig. 2is an enlarged view, partly broken away and in section, of the oathodeshown in Fig. 1, while Fig. 3 is' a fragmentary view, partly in section,of a modified form of cathode.

Referring to Fig. 1, numeral 1 designates the envelope of anelectricdischarge device which terminates in reentrant stems 2 and 3. The upperstem 2, as shown, is closed by an inverted stem and press 4 whichterminates in an open-ended skirt portion 5. The interior of the latterhas a transverse partition 6 provided with a tubular member '7 toexclude the glow discharge from the region of the press 4. A cylindricalcontrol or grid member 8, fabricated preferably of perfo rated oxidizednichrome is fitted tightly over the skirt portion and rigidly securedthereto by a screw clamp 10 which carries a lead 11 to the Serial No.487,541, filed Oct. 9, 1930, and assigned to the same assignee as thepresent application.

A metallic ring 14 is aflixed to the outer surface of the grid member toprevent a discharge from Y forming in the annular space between thereentrant stem 2 and the interior of the envelope.

Within the control member, there is a cylindrical graphite anode, (notshown), secured in place by a rod 15 which is sealed in the stem 4. v

The cathode 16 which forms the subject of the present invention issupported by a plurality of channels 17 welded to a clamp 18 whichtightly embraces the stem 3. There are two heat shields 19 afilxed tothe channels. The cathode illustrated in Fig. 1 is shown enlarged inFig. 2. The heater member 20 may consist of tungsten wound as a spiralon a rod 21 of insulation, for example, alumina (A1203). The upper endof the heater and its insulation support-are fixed into a metal hubmember 22 which is provided with a radial fiange 23. This flangesupports a plurality of cap members 24 which are spaced by metal rings25 and secured together by screws 26. The caps fit tightly over acorresponding number of openended perforated cylinders 28 which are heldapart by a plurality of spacers 29 hung on rivets. The lower ends of thecylinders are closed by metal members and have secured thereto a pair offlexible conductors 30 (Fig. 1) which are. joined to stiff wires 31sealed in the lower press. A similar combination of flexible and stifiwires is taken from the lower end of the heater member. Within thecylinders 28 (Fig. 2), there are a plurality of metal discs 32 inaddition to the heater 20 referred to. These discs.may be formed with astepped upset inner lip having a configuration such that the projectionof one disc fits snugly within the recess of the adjacent disc. Thediscs are rigidly secured together by a number of equidistantlyspaced'bolts 33 which also serve to hold the spacers 34 in place. Whenthe discs are fitted together and bolted, the cathode takes on a rigidself-supporting structure. The feature of providing discs withinterlockingv projections is described and claimedin the Pikeapplication, Serial No. 487,541, referred to hereinbefore.

The cylindrical members 28 serve as shields to conserve the heat energywhile theperforations therein allow ready egress for the electronsemitted by the discs'32. The latter are preferably coated with asubstance of low work function such as barium oxide which emitselectrons readily. The envelope 1 may be either highly evacuated orcontain inert gas or vapor, for example, mercury vapor, at a pressure,for example, 1 mm. mercury, sufficiently high to neutralize space chargeeffect thereby to support an arc discharge at practical voltages.

The operation-of a tube of this kind is well known and it is sufiicientto state that when alternating voltage is applied between anode andcathode, the grid member may be biased by alternating or direct currentto control the average rectified current in the output circuit. Thecontrol member may obviously be omitted, in which case the deviceperforms the function of a simple rectifier. It has been found duringoperation that a cathode provided with laterally extending discs givesadequate electron emission for an exceedingly large current.Furthermore, the increase of electron-emitting surface in the improvedcathode over the prior art cathodes is attained without undulyincreasing its mass or metal content so that the time taken by thecathode in reaching its normal operating temperature is not unduly long.

Fig. 3 shows a modified form of cathode in which the discs 35 are spunover at their inner edge to form a surface which may be readily weldedor otherwise secured to a metal cylinder 36. The cylinder is closed atthe top by a metal member 3'7 to which is fixed one end of a spiralheater 38. The other end of the spiral is attached to a metal rod fusedin a glass insulator 39. The lower end of the metal cylinder 36 may bethreaded into a metal cap 40 which carries the glass insulator and issealed to a ring of glass 41. This ring is fused to a metal cylinder 42which fits within the flanged mouth of an envelope 43. Inasmuch as thefilament is completely insulated from the envelope by the glass ring 41,the envelope may be made of metal and the cylinder 42 welded thereto.The envelope construction may take the form of a large size iron tankcapable of handling power currents of magnitude. The

cathode structure is exceedingly rugged and for a given mass presents alarge electron emitting surface to the anode by reason of its discconstruction. It is apparent that. if desired, heat shielding cylindersmay be provided as in the previous figure, further to enhance theoperating efficiency of the cathode.

Broad claims to the hollow or cavity cathode construction disclosedherein appear in my copending application, Serial No, 156,713, filedDec. 23, 1926. That application also contains claims relating to thecombination of a cathode and heat shielding means. The claims in thepresent application, therefore, are limited to improvements over theinvention described and claimed in the prior application referred toabove.

Claims directed to a hollow cathode provided with longitudinal vanes forincreasing the electron emitting surface appear in the Hull and Rugglesapplication, Serial No. 268,976, filed April 10, 1928.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A cathode for an electrical discharge device comprising a heatermember enclosed within a metal casing, said casing being provided with aplurality of discs extending transversely therefrom and adapted to emitelectrons, the size of the discs in said transverse direction beingconsiderably greater than that of the casing.

2. A cathode for an electrical discharge device comprising a heatermember enclosed within a metal casing, said casing being provided with aplurality of discs which extend normal to the surface of the casing andcoated with an electron emitting substance, the size of the discs insaid normal direction being considerably greater than that of thecasing.

3. A cathode for an electrical discharge device comprising a heatermember enclosed within a metal casing, discs secured to said casing andextending transversely therefrom, and one or more heat shieldssurrounding said discs. I

4. A cathode for an electrical discharge device comprising a heatermember enclosed within a metal casing, discs secured to said casing andextending transversely therefrom and one or more perforated cylinderssurrounding said discs.

5. A cathode for an electrical discharge device comprising a heatermember enclosed within a cylindrical metal casing, a plurality ofcircular electron-emitting discs secured directly to the casing, saiddiscs extending normal to the surface of the casing and having diametersat least several times the diametral size of the casing.

ALBERT W. HULL.

